This invention generally relates to methods of producing fuel for nuclear reactors wherein particulate uranium oxides are formed into pellets or integrated fuel units. The invention particularly relates to means for increasing the grain size of the particulate uranium oxide materials used in producing fuel by such methods.
The grain size of fissionable ceramic materials used in nuclear reactor fuels has been an important factor in the manufacture and the performance of nuclear reactor fuels. For instance, U.S. Pat. No. 3,883,623 proposes that particle size of ceramic uranium materials has an effect upon the density of fuel produced by sintering such particles, and discloses the significance of density in fuel products. On the other hand, U.S. Pat. No. 3,803,273 discloses an effect of the grain size in sintered composites or pellets of particulate fuel materials upon the performance of fuel elements in reactor service. In any case, it appears that grain size of fuel stocks influences significant aspects of fissionable ceramic fuels for nuclear reactors and therefore that the particle size contents or manipulation of this property of uranium oxide for use as fuel is of considerable consequence.
The manufacturing of ceramic fissionable fuel from powdered oxides of uranium is well known in the art, note for example U.S. Pat. Nos. 3,761,547; 3,803,273; 3,883,623; and 4,061,700, and the references cited therein. The disclosures of these patents showing the state of the art in this field are accordingly incorporated herein by reference.
Briefly, the manufacture of such ceramic fissionable fuel typically comprises cold pressing fine powdered oxides of uranium, either alone or admixed with other fissionable materials, into green coherent compacts or bodies such as pellets, and then sintering the green compacts of oxide powder to fuse the particles into an integrated mass.
Uranium dioxide is the principal source of fissionable fuel for commercial power generating nuclear reactors. Commercially available uranium dioxide is of a relatively fine particulate form comprising small particles or powder typically ranging up to about 10 to 15 microns in size, but averaging near 0.5 microns in size with no practical limit on the smaller sizes. The sintering operation typically increases grain size to a particle of about 20 microns. This relatively small particle size of commercial sources of the principal fuel material constitutes a marked disadvantage in the common fuel manufacturing process wherein large grain size is generally desired or beneficial, in addition to particle size having a significant influence upon other facets of the process or product thereof.